为什么这个简单的连接查询与子查询相比要快得多?

I need to join them on a column called order_number, which is a VARCHAR(50) on both. order_number is not unique in the outbound table.

CREATE TABLE `outbound` (
    `outbound_id` int(12) NOT NULL,
    `order_number` varchar(50) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

CREATE TABLE `order_details` (
    `order_details_id` int(12) NOT NULL,
    `order_number` varchar(50) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

这是我的初始查询，运行时间超过 60 秒:

SELECT o.order_number
FROM outbound o
INNER JOIN order_details od
    ON o.order_number = od.order_number

SELECT o.order_number
FROM outbound o
INNER JOIN
(
    SELECT order_number
    FROM order_details
) od
ON (o.order_number = od.order_number)